RE: [cgmo-webcgm] problems with NUBS

["Fuhr, Richard D" <richard.d.fuhr@pss.Boeing.com> (by way of Lofton Henderson <lofton@rockynet.com>)]

[...Forwarding for Richard, who can't write to the TC list yet...]

To answer your questions below.

1.  The proposed "better description" that I sent out last week (copied
below) was intended to merely be informatative and non-technical for the
purpose of communicating to the people on this mailing list, and would
not need to be incorporated into future versions of the CGM
specification itself. If someone had implemented B-splines per "CGM:1999
plus def1999_1-001", the proposed "better description" would not require
changes to implementation (viewer, authoring tool, etc)?

2.  The Defect Report Number 8632:1999-1/001 (a.k.a def1999_1-001) does
need to be incorporated in future versions of the CGM specification.

Regards,

Richard Fuhr

-----Original Message-----
From: Lofton Henderson [mailto:lofton@rockynet.com]
Sent: Sunday, September 04, 2005 11:33 AM
To: Fuhr, Richard D; cgmo-webcgm@lists.oasis-open.org
Subject: RE: [cgmo-webcgm] problems with NUBS

We're close on nailing the defect resolution(s).  One final question for
Richard.

At 09:55 AM 9/1/2005 -0700, Fuhr, Richard D wrote:
 >Here are my thoughts on the issues below.
 >
 >*.  While NUBS and NURBS are certainly not required for WebCGM 2.0,
 >they offer the following benefits. [...cut...]

I'm limiting this message to Defect correction.  We can handle
keep-remove decision separately.

 >[...]
 >1.  The ATA and WebCGM profiles should require clamped splines, since
 >the Model Profile does.

Everyone seems to agree here.

Although there is a transcription error in the MP column of the PPF, the
ATA profile (which allows nurbs) normatively specifies that the MP
values are per CGM:1999 Annex I (and the MP column of the ATA profile is
informative).  Same with WebCGM 1.0 (which didn't allow NURBS), and
WebCGM 2.0 2nd CD draft (which presently does allow them).

Plus, everyone *wants* clamped.


 >2.  We did indeed submit a defect report but it somehow did not get
 >incorporated into the spec.

Right.  I have attached a copy.  Dick Puk (SC24/WG6) says it will take
1-3 months to expedite this through SC24.

 >3.  In section 6.6.10.1.12 of ISO/IEC 8632-1 Second Edition 1999-12-15,

 >the recursive definition of B-splines uses the half-open interval T[i]
 ><= t < T[i+k] and I believe this is correct.

Okay, so considering only Rob's original comment about the divergence
between this current CGM:1999 text and the old defect report (1-065,
against the previous CGM:1992 text) ... strictly speaking, nothing needs
changing in CGM:1999 text, for accuracy and correctness.

Is that right?

 >     Regarding the interval over which the basis functions evaluate to
 > non-zero, it would be better to describe the property a bit
differently.

To clarify, is the proposed better description (see below):

1.) informative and non-technical?  [Editorial clarification]

2.) normative and technical?  [Technical defect fix.]

I.e., if someone had implemented B-splines per "CGM:1999 plus
def1999_1-001", would the proposed "better description" require changes
to implementation (viewer, authoring tool, etc)?


 >     Let J be the CLOSED interval = {t : T[i] <= t <= T[i+k]}.  Then
 > the B-spline basis function B[i,k] is zero OUTSIDE this interval, it
 > is positive in the OPEN interval K = {t : T[i] < t < T[i+k]} and it
 > may be either 0 or positive at the end points of the closed interval J

 > (this depends upon the particular basis function and the particular
knot sequence).

Can you phrase this as exact wording for a defect correction?  I.e.,
looking at CGM:1999, 6.6.10.1.2, it looks to me like you're addressing
the
4 equations that recursively define the basis functions B[i,k] (bottom
of
p.49 in my ISO edition of CGM:1999).

What, exactly, would change in those current basis function definitions?

 >     Informally speaking, for those B-spline basis functions whose
 > graphs look like bell-shaped curves, the values of the basis functions

 > are zero at each end of the closed interval J.  However, the first and

 > last basis functions defined over a clamped knot sequence (i.e., one
 > having knot multiplicity equal to degree+1 at the start and end of the

 > sequence) attain values of 1.0 at, respectively, the first and last
 > point of the interval J.


Regards,
-Lofton.